Mazda Speedometer Gears

Speedometer Gears

	Each gear at left is held in place by a 6 mm screw on the lower port rear side of a Mazda RWD transmission. The threaded part is where the speedometer cable attaches. You can see under each set of gear teeth where a roll pin goes through the housing. By removing the roll pin, the gear can be removed from the housing. These housings are one piece. Pre-'79 housings are multipart, with a separate retainer, but interchangable with one piece housings. The one on the left was removed from a 1987 RX-7 GXL and has 20 teeth. The one on the right was removed from a RWD 626 and has 22 teeth. By examining the picture carefully you can see two noteworthy things. One is that the two gears are different diameters. Two is that the gear shaft does not run through the center of the housing. Each is offset slightly, the smaller toward the output shaft, and the larger away from the output shaft. The offset allows the same gear inside the transmission to be used with different diameter and tooth count driven gears.
First generation RX-7s use a 5 tooth worm gear on the output shaft. 1986-up RX-7s use a 6 tooth worm gear on the output shaft. In the speedometer gear chart are listed 18, 19, 20, 21, and 22 tooth gears compatible with the '86-up 6 tooth worm. The only driven gear that fits the RX-7 5 tooth worm has 17 teeth. To change the worm, the shift mechanism and tailshaft housing must be removed from the transmission. This is not difficult if the transmission is out of the car. 18, 19 and 20 are the small diameter and use the housing pictured on the left. 21 and 22 are the large diameter and use the housing pictured on the right.

Speedometer Math

	Stock	Stock	What If
	12A	GSL-SE	Option
Speedometer RPM	1026	1026	1026
Mainshaft Teeth	5	5	6
Cable Teeth	17	17	22
Axle Ratio	3.909	4.077	4.300
Desired Tire RPM	892.38	855.65	874.88
Tire Size	185/70-13	205/60-14	225/50-15
Actual Tire RPM	894	874	868
Speedometer Error	0.18%	2.15%	-0.79%

In the chart, "Speedometer RPM" is the factory calibration. It is printed on the face of the guage, but it only shows when the guage is removed form its pod. For the factory speedometer to read correctly, the tires, rear axle ratio, and speedometer gears must combine to produce the calibration RPM. So in the chart, using the last column as example, divide the calibration RPM, 1026, by both the mainshaft teeth, 6, and the axle ratio, 4.3, and multiply that result by the teeth on the cable gear, 22, to produce the tire RPM that results in theoretical 100% speedometer accuracy, 874.88.

To try to obtain a more accurate speedometer reading after an axle ratio or tire size change, simply substitute unknowns. For example. you have already installed a 6 tooth worm and 205/55-14 tires and want to know the best cable gear to install. RPM for that tire size is 910. Using stock SE gears, divide axle ratio, 4.077, by speedo RPM, 1026, and multiply by tire RPM, 910, and multiply by worm teeth, 6, producing 21.696. The closest fit is 22. Using a 22 tooth gear, we calculate theoretical error with our combination as follows: Divide 1026 by 6 and by 4.077, and multiply by 22, giving target tire RPM of 922.737. Divide 922.737 by actual tire RPM of 910 to get the error of 1.4% pessimistic (actual tire RPM less than target RPM makes speedo read low).

If you are interested in highest possible accuracy, a trip to a speedometer shop is probably required. They can supply and/or install a small gearbox that installs between cable and cable gear, providing the speed adjustment required to account for whatever you have changed or contemplate changing, or to remove any error built in at the factory.

Images courtesy of John Sisler

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